schaeffner.ppt
- 1. Multiple Access Techniques for Wireless Communication FDMA TDMA SDMA PDMA A Presentation by Schäffner Harald
- 2. Introduction • many users at same time • share a finite amount of radio spectrum • high performance • duplexing generally required • frequency domain • time domain
- 3. Frequency division duplexing (FDD) • two bands of frequencies for every user • forward band • reverse band • duplexer needed • frequency seperation between forward band and reverse band is constant frequency seperation reverse channel forward channel f
- 4. Time division duplexing (TDD) • uses time for forward and reverse link • multiple users share a single radio channel • forward time slot • reverse time slot • no duplexer is required time seperation t forward channel reverse channel
- 5. Multiple Access Techniques • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Code division multiple access (CDMA) • Space division multiple access (SDMA) • grouped as: • narrowband systems • wideband systems
- 6. Narrowband systems • large number of narrowband channels • usually FDD • Narrowband FDMA • Narrowband TDMA • FDMA/FDD • FDMA/TDD • TDMA/FDD • TDMA/TDD
- 7. Logical separation FDMA/FDD f t user 1 user n forward channel reverse channel forward channel reverse channel ...
- 8. Logical separation FDMA/TDD f t user 1 user n forward channel reverse channel forward channel reverse channel ...
- 9. Logical separation TDMA/FDD f t user 1 user n forward channel reverse channel forward channel reverse channel ...
- 10. Logical separation TDMA/TDD f t user 1 user n forward channel reverse channel forward channel reverse channel ...
- 11. Wideband systems • large number of transmitters on one channel • TDMA techniques • CDMA techniques • FDD or TDD multiplexing techniques • TDMA/FDD • TDMA/TDD • CDMA/FDD • CDMA/TDD
- 12. Logical separation CDMA/FDD code f user 1 user n forward channel reverse channel forward channel reverse channel ...
- 13. Logical separation CDMA/TDD code t user 1 user n forward channel reverse channel forward channel reverse channel ...
- 14. Multiple Access Techniques in use Multiple Access Technique Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Cellular System
- 15. Frequency division multiple access FDMA • one phone circuit per channel • idle time causes wasting of resources • simultaneously and continuously transmitting • usually implemented in narrowband systems • for example: in AMPS is a FDMA bandwidth of 30 kHz implemented
- 16. FDMA compared to TDMA • fewer bits for synchronization • fewer bits for framing • higher cell site system costs • higher costs for duplexer used in base station and subscriber units • FDMA requires RF filtering to minimize adjacent channel interference
- 17. Nonlinear Effects in FDMA • many channels - same antenna • for maximum power efficiency operate near saturation • near saturation power amplifiers are nonlinear • nonlinearities causes signal spreading • intermodulation frequencies
- 18. Nonlinear Effects in FDMA • IM are undesired harmonics • interference with other channels in the FDMA system • decreases user C/I - decreases performance • interference outside the mobile radio band: adjacent-channel interference • RF filters needed - higher costs
- 19. Number of channels in a FDMA system • N … number of channels • Bt … total spectrum allocation • Bguard … guard band • Bc … channel bandwidth N= Bt - Bguard Bc
- 20. Example: Advanced Mobile Phone System • AMPS • FDMA/FDD • analog cellular system • 12.5 MHz per simplex band - Bt • Bguard = 10 kHz ; Bc = 30 kHz N= 12.5E6 - 2*(10E3) 30E3 = 416 channels
- 21. Time Division Multiple Access • time slots • one user per slot • buffer and burst method • noncontinuous transmission • digital data • digital modulation
- 22. Slot 1 Slot 2 Slot 3 … Slot N Repeating Frame Structure Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.
- 23. Features of TDMA • a single carrier frequency for several users • transmission in bursts • low battery consumption • handoff process much simpler • FDD : switch instead of duplexer • very high transmission rate • high synchronization overhead • guard slots necessary
- 24. Number of channels in a TDMA system • N … number of channels • m … number of TDMA users per radio channel • Btot … total spectrum allocation • Bguard … Guard Band • Bc … channel bandwidth N= m*(Btot - 2*Bguard) Bc
- 25. Example: Global System for Mobile (GSM) • TDMA/FDD • forward link at Btot = 25 MHz • radio channels of Bc = 200 kHz • if m = 8 speech channels supported, and • if no guard band is assumed : N= 8*25E6 200E3 = 1000 simultaneous users
- 26. Efficiency of TDMA • percentage of transmitted data that contain information • frame efficiency f • usually end user efficiency < f , • because of source and channel coding • How get f ?
- 27. Slot 1 Slot 2 Slot 3 … Slot N Repeating Frame Structure Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.
- 28. Efficiency of TDMA • bOH … number of overhead bits • Nr … number of reference bursts per frame • br … reference bits per reference burst • Nt … number of traffic bursts per frame • bp … overhead bits per preamble in each slot • bg … equivalent bits in each guard time intervall bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
- 29. Efficiency of TDMA bT = Tf * R • bT … total number of bits per frame • Tf … frame duration • R … channel bit rate
- 30. Efficiency of TDMA • f … frame efficiency • bOH … number of overhead bits per frame • bT … total number of bits per frame f = (1-bOH/bT)*100%
- 31. Space Division Multiple Access • Controls radiated energy for each user in space • using spot beam antennas • base station tracks user when moving • cover areas with same frequency: • TDMA or CDMA systems • cover areas with same frequency: • FDMA systems
- 32. Space Division Multiple Access • primitive applications are “Sectorized antennas” • in future adaptive antennas simultaneously steer energy in the direction of many users at once
- 33. Reverse link problems • general problem • different propagation path from user to base • dynamic control of transmitting power from each user to the base station required • limits by battery consumption of subscriber units • possible solution is a filter for each user
- 34. Solution by SDMA systems • adaptive antennas promise to mitigate reverse link problems • limiting case of infinitesimal beamwidth • limiting case of infinitely fast track ability • thereby unique channel that is free from interference • all user communicate at same time using the same channel
- 35. Disadvantage of SDMA • perfect adaptive antenna system: infinitely large antenna needed • compromise needed
- 36. SDMA and PDMA in satellites • INTELSAT IVA • SDMA dual-beam receive antenna • simultaneously access from two different regions of the earth
- 37. SDMA and PDMA in satellites • COMSTAR 1 • PDMA • separate antennas • simultaneously access from same region
- 38. SDMA and PDMA in satellites • INTELSAT V • PDMA and SDMA • two hemispheric coverages by SDMA • two smaller beam zones by PDMA • orthogonal polarization
- 39. Capacity of Cellular Systems • channel capacity: maximum number of users in a fixed frequency band • radio capacity : value for spectrum efficiency • reverse channel interference • forward channel interference • How determine the radio capacity?
- 40. Co-Channel Reuse Ratio Q • Q … co-channel reuse ratio • D … distance between two co-channel cells • R … cell radius Q=D/R
- 41. Forward channel interference • cluster size of 4 • D0 … distance serving station to user • DK … distance co-channel base station to user
- 42. Carrier-to-interference ratio C/I • M closest co-channels cells cause first order interference C = I D0 -n0 M k=1 DK -nk • n0 … path loss exponent in the desired cell • nk … path loss exponent to the interfering base station
- 43. Carrier-to-interference ratio C/I • Assumption: • just the 6 closest stations interfere • all these stations have the same distance D • all have similar path loss exponents to n0 C I = D0 -n 6*D -n
- 44. Worst Case Performance • maximum interference at D0 = R • (C/I)min for acceptable signal quality • following equation must hold: 1/6 * (R/D) (C/I)min = > -n
- 45. Co-Channel reuse ratio Q • D … distance of the 6 closest interfering base stations • R … cell radius • (C/I)min … minimum carrier-to-interference ratio • n … path loss exponent Q = D/R = (6*(C/I)min) 1/n
- 46. Radio Capacity m • Bt … total allocated spectrum for the system • Bc … channel bandwidth • N … number of cells in a complete frequency reuse cluster m = Bt Bc * N radio channels/cell
- 47. Radio Capacity m • N is related to the co-channel factor Q by: Q = (3*N) 1/2 m= Bt Bc * (Q²/3) = Bt Bc * 6 C I 3n/2 ( *( )min ) 2/n
- 48. Radio Capacity m for n = 4 • m … number of radio channels per cell • (C/I)min lower in digital systems compared to analog systems • lower (C/I)min imply more capacity • exact values in real world conditions measured m = Bt Bc * 2/3 * (C/I)min
- 49. Compare different Systems • each digital wireless standard has different (C/I)min • to compare them an equivalent (C/I) needed • keep total spectrum allocation Bt and number of rario channels per cell m constant to get (C/I)eq :
- 50. Compare different Systems • Bc … bandwidth of a particular system • (C/I)min … tolerable value for the same system • Bc’ … channel bandwidth for a different system • (C/I)eq … minimum C/I value for the different system C I = C I Bc Bc’ ( ) ( ) min )² eq * (
- 51. C/I in digital cellular systems • Rb … channel bit rate • Eb … energy per bit • Rc … rate of the channel code • Ec … energy per code symbol C Eb*Rb Ec*Rc I I I = =
- 52. C/I in digital cellular systems • combine last two equations: (C/I) (Ec*Rc)/I Bc’ (C/I)eq (Ec’*Rc’)/I’ Bc = = ( )² • The sign ‘ marks compared system parameters
- 53. C/I in digital cellular systems • Relationship between Rc and Bc is always linear (Rc/Rc’ = Bc/Bc’ ) • assume that level I is the same for two different systems ( I’ = I ) : Ec Bc’ Ec‘ Bc = ( )³
- 54. Compare C/I between FDMA and TDMA • Assume that multichannel FDMA system occupies same spectrum as a TDMA system • FDMA : C = Eb * Rb ; I = I0 * Bc • TDMA : C’ = Eb * Rb’ ; I’ = I0 * Bc’ • Eb … Energy per bit • I0 … interference power per Hertz • Rb … channel bit rate • Bc … channel bandwidth
- 55. Example • A FDMA system has 3 channels , each with a bandwidth of 10kHz and a transmission rate of 10 kbps. • A TDMA system has 3 time slots, a channel bandwidth of 30kHz and a transmission rate of 30 kbps. • What’s the received carrier-to-interference ratio for a user ?
- 56. Example • In TDMA system C’/I’ be measured in 333.3 ms per second - one time slot C’ = Eb*Rb’ = 1/3*(Eb*10E4 bits) = 3*Rb*Eb=3*C I’ = I0*Bc’ = I0*30kHz = 3*I • In this example FDMA and TDMA have the same radio capacity (C/I leads to m)
- 57. Example • Peak power of TDMA is 10logk higher then in FDMA ( k … time slots) • in practice TDMA have a 3-6 times better capacity
- 58. Capacity of SDMA systems • one beam each user • base station tracks each user as it moves • adaptive antennas most powerful form • beam pattern G() has maximum gain in the direction of desired user • beam is formed by N-element adaptive array antenna
- 59. Capacity of SDMA systems • G() steered in the horizontal -plane through 360° • G() has no variation in the elevation plane to account which are near to and far from the base station • following picture shows a 60 degree beamwidth with a 6 dB sideslope level
- 60. Capacity of SDMA systems
- 61. Capacity of SDMA systems • reverse link received signal power, from desired mobiles, is Pr;0 • interfering users i = 1,…,k-1 have received power Pr;I • average total interference power I seen by a single desired user:
- 62. Capacity of SDMA • i … direction of the i-th user in the horizontal plane • E … expectation operator I = E { G(i) Pr;I} K-1 i=1
- 63. Capacity of SDMA systems • in case of perfect power control (received power from each user is the same) : Pr;I = Pc • Average interference power seen by user 0: I = Pc E { G(i) } K-1 i=1
- 64. Capacity of SDMA systems • users independently and identically distributed throughout the cell: I = Pc *(k -1) * 1/D • D … directivity of the antenna - given by max(G()) • D typ. 3dB …10dB
- 65. Pb = Q ( ) Capacity of SDMA systems • Average bit error rate Pb for user 0: 3 D N K-1 • D … directivity of the antenna • Q(x) … standard Q-function • N … spreading factor • K … number of users in a cell
- 66. Capacity of SDMA systems